An electrical signal carried on a wire generates electrical and magnetic fields in proximity to the wire along its entire length. These fields extend beyond the wire's copper conductor and through its insulation into the surrounding space. If other wires are near the generating wire, the fields will extend through their insulations as well, coming in contact with their conductors. Electrical and magnetic interactions will occur, generating new currents on the other wires. This phenomenon is termed “crosstalk” and is normally considered detrimental to the operation of the affected circuits. However, at low levels or specific electrical frequencies, crosstalk is often inconsequential, depending upon the nature of the “victim” signal.
Shielding for individual wires or pairs of wires is often built into cables to reduce and control crosstalk. Wires used for sensitive signals are usually shielded, and noisy circuits are unshielded as opposed to shielding both, which would unnecessarily increase weight and cost beyond the requirement for shielding the sensitive lines.
Circuits are sensitive to a threat from other circuits on a graduated basis. It is therefore possible to arrange circuits in, for example, a flat ribbonized array, and assign specific circuits to specific positions (a good method is from highest power to lowest power) to minimize the average coupling of the circuits. The number of wire positions separating circuits directly influences their crosstalk. The more distance between wires, the less the crosstalk. Under the proper circuit parameters, this organization may also provide a crosstalk situation to which all of the circuits are tolerant, allowing safe and proper operation of the electrical equipment hooked up to the wires without using shielding on any of the wires or pairs.
This organizing of circuits on flat ribbons without the utilization of shielded wires is one of the main technical practices of Ribbonized Organized Integrated (ROI) wiring, an “Organized Wiring” methodology used in many military and some commercial aircraft wiring systems. ROI ribbon harnesses use as many as six or even more woven wire ribbons stacked in a pack, separated by electrically grounded copper foils and normally covered by a braided shield. The foils between ribbons reduce coupling from ribbon to ribbon, and the braided shield prevents sources of interference outside of the harness from causing crosstalk effects.
Some types of electrical cables include a number of separate conductors that carry a number of different types of electrical signals. (As used herein, a “cable” has a generally round cross section, as distinct from the flat cross section of the ribbon.) In an example of interest, an in-flight entertainment (IFE) system in an airliner may include, at each seat, a television with headphone connections, an electrical connection, a telephone connection, and a data port. Such an in-flight entertainment system requires a video signal, an audio signal, a power signal, a telephone signal, data signals, and control signals at each seatback. Some of these signals may be multiplexed and share the same transmission wires. All of these electrical signals are carried on wires that are bundled into an IFE electrical cable for compactness, neatness, and convenience in installation and maintenance.
Some of the bundled wires in the electrical cable carry electrical signals that may interfere with each other or with the other electrical functionality of the aircraft, or which may be interfered with by other electrical signals in the aircraft. To prevent such interference, some of the wires are shielded with a grounded metallic shield, and the exterior of the entire cable may be shielded with another grounded metallic shield. Such shielding adds weight, bulk, and cost to the electrical cable. The physical spaces allocated to the electrical cables are tightly constrained. In some cases the sizes of the electrical conductors within the electrical cable must be made smaller than desired in order to fit within the allocated spaces, taking into account the presence of the shielding and the insulation, or shielding is limited or removed. The result is that the performance of the IFE system is compromised. This background discussion has focused on IFE systems, but the same problems arise in other types of aircraft and other electrical cable systems.
There is a need for an improved approach to electrical cables that must carry different types of electrical signals and are constrained in weight and/or size. The present invention fulfills this need, and further provides related advantages.